1. Field of the Invention
The invention relates to a wheeled inverted pendulum mobile unit.
2. Description of the Related Art
A “wheeled inverted pendulum mobile unit” in the specification is a wheeled mobile unit that runs with a single driving wheel or a plurality of coaxially arranged driving wheels, and means a mobile unit that runs while keeping the balance of the body that is tiltable about the axle(s) of the driving wheel(s). In the specification, the “wheeled inverted pendulum mobile unit” is simply referred to as “mobile unit” as needed.
The center of gravity of the body of the wheeled inverted pendulum mobile unit is located above the axle(s) of the driving wheel(s). Thus, the wheeled inverted pendulum mobile unit is unstable unless the balance of the mobile unit is controlled. The “unstable” means that, while keeping the balance of the body, a small disturbance causes the body to lose its balance. Thus, all state quantities (state functions) or a portion of state quantities of the body and wheel(s) are subjected to a feedback control to stabilize the posture of the body (tilting angle of the body with respect to a vertical direction). This control is termed the “inverted pendulum control” in the specification. To be simple, a control for maintaining the tilting angle of the body by driving the driving wheels is termed as the “inverted pendulum control”.
The wheeled inverted pendulum mobile unit is an unstable system in terms of control. That is, if the inverted pendulum control stops, the body falls. Alternatively, if the inverted pendulum control malfunctions, the body falls. Thus, the wheeled inverted pendulum mobile unit includes auxiliary support members in order to prevent the body from falling. For example, Japanese Patent Application Publication No. 2006-247802 (JP-A-2006-247802) describes a moving robot. The moving robot is equipped with support legs, which can be raised or lowered, and is controlled through an inverted pendulum control. During the inverted pendulum control, the moving robot keeps the support legs away from the ground. When the inverted pendulum control is stopped or when the inverted pendulum control malfunctions because of an unexpected event, the moving robot lowers the support legs to contact the ground. Thus, the lowered support leg supports the body to prevent the body from falling.
The support legs of the moving robot described in JP-A-2006-247802 does not contact the ground during the inverted pendulum control. At the time the inverted pendulum control is stopped or at the time the inverted pendulum control malfunctions, the moving robot lowers the support leg. Thus, there is a time lag between the time the inverted pendulum control malfunctions and the time the support leg contacts the ground. During the time lag, the body is unstable and, therefore, variation in the tilting angle of the body may occur. In addition, when the wheeled inverted pendulum mobile unit carries a person, the time lag, during which the body cannot be controlled at a constant tilting angle, makes the rider feel uneasy. Not only in the occurrence of an unexpected event but also when the inverted pendulum control is stopped, a time lag between the time the inverted pendulum control is stopped and the support leg contacts the ground causes a similar problem.